Plastic dielectric composition and shielded spark plug terminal construction comprising same



Get. 7, 1947. s. L. BASS PLASTIC DIELECTRIC COMPOSITION AND SHIELDED SPARK PLUG TERMINAL CONSTRUCTION COMPRISING SAME Filed Nov. 2, 1942 6 I F a m p, M m w 4 w W m H W N C 6 m 5 i Patented Oct. 7, 1947 UNITED PLASTIC DIELECTRIC COMPOSITION AND SHIELDED SPARK PLUG TERMINAL CON- STRUCTION COMPRISING SAME Shailer L. Bass, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich, a

corporation of Michigan Application November 2, 1942(, Serial No. 464,320

This invention concerns certain new dielectric compositions which remain soft and plastic over a wide range of temperatures and which are adapted to use as agents for sealing and/or insulating high tension cables or other conductors, especially at points where they lie near, e. g., within one inch of, a grounded metal, so as to prevent the occurrence of an electric corona. The invention also concerns electric conductors when so sealed or insulated, and it particularly concerns the sealing of high tension cables, e. g., a terminal of an ignition cable where the latter fits into and connects with a shielded spark plug. The term corona is herein employed in the usual sense of referring to an electric discharge or leakage of current due to conductance (presumably ionic conductance) by a gas, as distinguished from the occurrence of an abrupt electric are or spark across the gap between oppositely charged conductors due to a large difference in electric potential of the conductors.

The spark plugs for aircraft engines usually include a grounded metal shield which extends upward from the plug proper and surrounds the ignition cable terminal where it connects with the central electrode of the plug. The terminal end or the ignition cable is fitted with a sleeve or thimble of a solid dielectric material, e. g.. a ceramic material, which sleeve or thimble fits fairly snugly inside of the tubular shield and serves as a non-conductive spacer for maintaining the terminal end of the cable in a central position. Although the space between the conductor member of the cable and the grounded metal shield may largely be filled by the insulating sleeve, the parts are mechanically fitted and there are voids, or air spaces, between the central conductor and the shield.

Serious difliculties have been encountered during use of such shielded ignition system, which have been the cause of failures of engines, particularly at high altitudes. For example, there has occurred fairly rapid corrosion and other damage to ignition cables where they connect with the spark plugs, such as to necessitate frequent repair or replacement of the cables, and damage near the terminal end of the cables to the covering of rubber or other flexible insulation over the same.

There are a number of contributing causes for these difliculties, such as the diificulty or completely sealing against air and moisture the end of the spark plug terminal shield; the high temperature of the spark plug during use; a tendency for moisture to collect along the ignition 11 Claims. (Cl. 174-35) cable, especially at the spark plug terminal; and the occurrence of slight leakage of gases from the combustion chamber into the well of the terminal shield on the plug. The principal and primary cause for the difilculties, however, appears to be the occurrence of corona discharge within the terminal chamber in the space between the grounded metal shield and the central electric conductor. Such corona becomes more intense as the gas pressure is reduced and in the rarified atmosphere prevailing at high altitudes may amount to a short circuit. This corona apparently causes a chemical reaction between nitrogen and oxygen of the air to form nitrogen oxides, which in turn react with the accumulated moisture to form nitric acid, since cables which have failed where they connect with the spark plugs have been found to be corroded at the terminal Junction, as if by acid, and nitric acid has been detected in the well of the terminal shield. For these and other reasons, it appears that the occurrence of corona discharge is the principal cause of the dimculties which have been encountered in using spark plugs having shielded terminal connections with the ignition cables, and that in the absence of such corona, the other contributing causes mentioned above would be of relatively little consequence.

Occurrence of the corona may be prevented by filling the space between the metal shield and the central electric conductor with a non-gaseous dielectric material. However, to be satisfactory for this purpose, a dielectric material must meet a number of requirements, via, (1) it must possess satisfactory dielectric properties, e. g. a di-.

electric strength of more than 2500 volts at 0.010 inch spark gap; (2) it must be chemically inert to the metal parts with which it is contacted; (3) it must retain its dielectric properties during prolonged use at fairly high temperatures, e. g., at temperatures of from 300 to 400F.; (4) it must be fairly thick or viscous and at the same time be sufificiently plastic to permit filling of the terminal chamber by hand, even at temperatures far below 0 F. and as low as 40 F.; (5) it must change only moderately, and preferably not at all, in consistency or viscosity over a wide range of temperatures, e. g., a temperature range of from 40 to 400 F.; (6) it should not burn or flash at temperatures below 500 F.; and (7) it should be non-hygroscopic and should be substantially impermeable to moisture.

Although dielectric agents are known which possess some of these necessary properties, none of the previously known dielectric agents possess 3 the full combination of properties required for this P p It is an object of this invention to provide new dielectric compositions which do possess the above-stated combination of properties and which are, accordingly} well suited to use not only as agents for sealing. the shielded terminals of spark plugs. but also as agents for covering or sealing other electric conductors or terminals, particularly where the part to be covered or sealed is to be used for the transmission of electric currents of high voltage at widely varying temperatures and in a position closely adjacent to a grounded metal.

We have found that a finely divided inorganic aerogel, e. g.. of silicon dioxide, aluminum oxide, or iron oxide or a mixture of such oxides, may be mixed, as by grinding, in suitable proportions with liquid polymers of organo-silicones to obtain homogeneous mixtures which are far more viscous than the polymers alone and are well adapted for use in sealing the space between an electric conductor and an adjacent grounded metal. An unexpected characteristic of such mixtures, which make them especially adaptable to the above use,'is the retention of their smooth Vaseline-like consistency even when heated up to 600 1"., that is, the liquid organo-silicone polymers do not flow away from the aerogel filler.

As is well known, an aerosel is a gel of finely porous structure such as may be obtained by displacing water from a liydrogel in such manner as to avoid excessive shrinking or crushing of the cellular structure of the body. Ways of preparing aerogels aredescribed by S. 8. Kistler in J. Phys. Chem. 36 52 (1932), and aerogels prepared in such usual ways are preferably used in our new dielectric compositions. However, this invention is not restricted as to the source or preparation of the aerogel. For instance, the fluify silica ash obtained by thoroughly burning in the vapor phase any volatile silicon compound, e, g. silicon tetrachloride vapor, fed into an oxygen-illuminating gas flame is highly porous and has an absorptive capacity for the liquid organo-silicone polymers which approaches that of a silica aerogel prepared in the usual ways. This silica ash behaves like the silica aerogel mentioned above and it is regarded as such for purpose of this invention.

The viscosity of the mixture of an aerogel and an organo-silicone polymer is dependent both on the viscosity of the polymer employed in preparing the same and upon the proportion of aerogel admixed with the polymer. In general, the proportion of aerogel required to obtain a mixture of given viscosity, or consistency, becomes less with'increase in the viscosity of th polymer to be admixed therewith. In practice, we preferably employ silica aerogel and a liquid organosilicone polymer oi' viscosity greater than 500 Saybolt seconds at 100 F. in preparing the dielectric compositions, in which case the addition to the polymer of from 3 to per cent by weight of the aerosol is usually suiiicient to obtain a mixture having the consistency of a soft grease. The addition of a smaller amount of the aerogel, e. g., as little as 0.1 per cent, to a liquid silicone polymer is suflicient to cause a marked increase in the viscosity of the latter. The resultant liquid dielectric composition, of viscosity higher than that of the polymer alone, is also useful as a liquid dielectric agen't in high tension cables.

Liquid organo-silicone' polymers which are adapted to the preparation of the new dielectric compositions may be obtained by the hydrolysis and the chemical condensation of one or more hydrolyzable silicon compounds having the general formula RzSlXa, wherein R. is a lower alkyl radical and x in a hydrolyzable group selected from the class consisting of halogen and alkoxy groups. They may also be obtained by the hydrolysis and chemical condensation of a mixture of alkylated silicon compounds containing at least '15 mole per cent of such dialkyl silicon compound having the general formula RaSiX: and not more than 25 mole per cent of a mono-alkyl silicon compound having the formula RSiXa, or a total of not more than 25 mole per cent of both such mono-alkyl silicon compound having the ,formula RSiX: and a trialkyl silicon compound having the formula RaSiX; in all oi which formulas R and K have the meanings stated above. In general, the hydrolyzable silicon compound or mixture of such compounds from which the liquid organo-silicone polymers may be prepared is one having an average composition corresponding to the formula B's-Six. wherein z is a whole or a fractional number from 2.25 to 1.5 and R and x have the meanings given above.

Examples of hydrolyzable dialkyl silicon compounds which may be used in preparing the liquid organo-silicone polymers are dimethyl silicon dichloride, methyl ethyl sllicon dichloride, diethyl silicon dichloride, methyl propyl silicon dichloride, dimethyl silicon dibromide, diethyl silicon dibromide, dimethyl-dimethoxy-silicon, diethyldiethoxy-silicon. dimethyl-diethoxy-silicon, etc. Examples of hydrolyzable mono-alkyl silicon compounds and hydrolyzable trialkyl silicon compounds which may be present together with the dialkyl silicon compound in amount not exceeding 25 mole per cent of the mixture are methyl silicon trichloride, ethyl silicon tribromide, ethyl silicon trichloride, propyl silicon trichloride, methyl-trimethoxy-silicon, methyl-triethoxy-silicon, ethyl-triethoxy-silicon, trimethyl silicon chloride, trimethyl silicon bromide, triethyl silicon chloride, trimethyl-methoxy-silicon, trinzethyl-ethoxy-silicon, triethyl ethoxy silicon, e c.

The liquid organo-silicone polymer may be obtained by heating the hydrolyzable silicon compound orcompounds with water in the presence of a hydrolysis catalyst, e. g., a mineral acid. HY- drolysis of the silicon compound to form corresponding organo-silicols (which silicols are unstabl under the reaction conditions and in some instances have not been isolated as such) is accompanied by chemical condensation of the siliccls to form the liquid organo-silicone polymer (or co-polymer) product. The starting materials are selected so that the product contains an average of between 1.75 and 4, and preferably between 1.9 and 2.5, atoms of carbon per atom of silicon.

The viscosity of suchpolymer or co-polymer is, of course, dependent upon the reaction conditions employed in preparing the same, e. g., the polymers of dimethyl silicone vary from thin liquids to viscous liquids to solid resins depending upon the conditions under which they are prepared. As hereinbefore mentioned, it is the liquid polymers and copolymers having a viscosity exceeding 500 Saybolt seconds at R, which are usually employed in preparing th new dielectric compositions and such liquid polymers of dimethyl silicone are preferred.

In preparing the dielectric compositions, the aerogel is preferably employed in amount sumcient to form with the silicone polymer a highly viscous and preferably paste-like mixture having the consistency of a soft grease, e. g., a consistency such as that of Vaseline. when using a silicone polymer of viscosity greater than 500 Saybolt seconds at 100 F., the proportion of silica aerogel is usually between 3 and per cent by weight and in most instances between 5 and 7 per cent, but the composition may contain the silica aerogel in proportions somewhat greater than those Just stated, e. g., as high as per cent. It may be mentioned that liquid silicone polymers of viscosity as high as 38,000 Saybolt seconds have been satisfactorily used in preparing the dielectric compositions.

The procedure in preparing the dielectric compositions should be such as to saturate the aerogel as fully as possible with the liquid silicone polymer. This is usually accomplished by mixing the aerogel and the polymer in the desired proportions on compounding rolls. The mixture may be heated, e. g., at temperatures of from 250 to 350 F., to facilitate absorption of the polymer by the aerogel.

We have found that the mixtures thus prepared possess an unusual combination of properties which render them well suited as agents for sealing the shielded ignition cable terminal of a spark plug. The mixtures are suillciently plastic to permit manual application oi! the same to the terminal even at temperatures as low as 40' F. They do not change greatly in consistency or body over a temperature range of from 40 F. to 400 F. When of a consistency such as that of Vaseline, the mixtures do not readily exude the liquid polymer upon being heated, e. g., to 250-400 F., and neither is the liquid lost to appreciable extent by evaporation. The mixtures possess good dielectric properties, but do not prevent satisfactory electric contact between the central electrode of a spark plug and the ignition cabl terminal when the contact is made by forcing said terminal through a body of the mixture covering the electrode; The mixtures are substantially non-flammable, i. e., the flash and fire points of the same are far higher than the temperatures to which they are heated during use. They are noncorrosive to copper, iron, and other structural metals and non-injurious to natural and synthetic rubbers.

The dielectric mixtures possess the unusual property that their dielectric properties are not reduced appreciably by passage of an electric arc therethrough. When an electric are or spark is passed through most solid or paste-like organic substances, a carbon track, resulting from decomposition of the carbon-containing compound, remains. This carbon track acts as a conductor for the passage of subsequent charges of electricity through the substance. However, when an electric spark is passed through one of the mixtures of the present invention, an electrically non-conductive grey track, apparently of silica or aerogel, appears. In order to avoid th possibility of occurrence of a carbon track upon passage of an electric spark through the mixture, it is important, however, that the atomic ratio of carbon to silicon in the organo-silicone polymer not exceed 4. As hereinbefore mentioned the liquid organo-silicone polymer employed in the dielectric composition is preferably one having a ratio of carbon atoms to silicon atoms of between 1.9 and 2.5.

The dielectric composition is useful not only in sealing the shielded ignition cable terminal of a spark plug, but in shielding any conductor for electric currents of high voltage at points where the conductor is positioned quite near a grounded metal, e. g., the composition may be used to seal the connection between the ignition cable and a magneto. The dielectric composition may be stored or shipped in usual containers, e. g., in cans or in collapsible tubes.

The application of the new dielectric compositions in sealing the shielded ignition cable terminal of a spark plug is illustrated in the accompanying drawing which is a cross-sectional side view of an ignition cable assembled with a spark plug having a shield for the cable terminal. In the drawing, the numeral I designates the metallic shell of a spark plug, which shell is provided with an upward tubular extension 2 that serves as a shield for the ignition cable terminal. In the drawing, the shell I and shield 2 are shown as integral, 1. e., constructed as one piece, but in practice they are often constructed as separate pieces which are threaded so as to permit assemblage. The shell I is provided near its lower end with outside threads for engaging the cylinder head and with inward projecting prongs 3 which serve as electrodes. The central electrode 4 of the spark plug is separated and electrically insulated from shell I and shield 2 by a usual insulator 5 which may be of a ceramic material, such as sillimanite. Within shell i and around a section of the insulation 5 is a band 8 of copper or brass which serves as a thermal expansion member. The shield 2 is threaded near its upper end and is fitted with a coupling member I. Extending above shield 2 and coupled with the latter by means of the member I is a curved metal tube 0 which is threaded at its outer end and which serves as a curved extension of the tubular shield 2. Within shield 2 is a tubular insulating sleeve 9. The latter, as shown, is constructed in sections, the upper section being of a natural or synthetic rubber and the lower section being constructed of ceramic material and being provided .with an electric contact member ill in the form of a spring which makes contact with the central electrode 4 of the spark plug. However, the insulating sleeve 9 may be constructed as a single tubular piece of any solid insulating material such as a ceramic material, a phenol-formaldehyde resin. or paper impregnated with such resin, etc. An ignition cable ll extends through the bore of tube 8 and sleeve 9. The cable comprises a central conductor l2, a surrounding layer iii of a flexible electric insulation such as natural or synthetic rubber, and a lacquered fabric covering ll. At its terminal end the cable ll makes electric contact with a metal washer I! to which the spring contact member I0 is attached. The outer end of the curved tube 8 is closed by a washer i of rubber or other resilient material, which washer fits tightly about the cable II. A cap H, which is screwed onto the outer end of tube I and over the washer I8, is provided with a, central opening of size sufiicient to receive the end of a braided metal conduit or harness it through which the cable is passed. The spaces is between the cable II or conductor l2 and the metal tubes 2 and I are those within which the electric corona, principally responsible for the operating difficulties hereinbeiore mentioned, usually occur.

The occurrence of such corona is prevented, according to this invention, by filling the spaces I. with a viscous or paste-like homogeneous colloidal mixture of a liquid organo-silicone polymer and an inorganic aerogel. The mixture is readily applied, merely by placing the same in the lower end of the bore of the tubular shield 2, shoving the terminal end of cable I I (which bears the insulating sleeve 0. the tube 8, coupling member I, washer l6. and cap I! in loosely assembled position) into place within the shield so that the spring contact member ID is seated in contact with the central electrode 4, and tightening the coupling member 1 and the cap l1. By such operations the viscous or paste-like dielectric mixture is caused to flow within and fill the spaces IS.

The following example describes one way in which the principle of the invention has been applied, but is not to be construed as limiting the invention.

Example A mixture of 95 parts by weight oi a liquid polymer of dimethyl silicone having a viscosity of 1000 Saybolt seconds at 100 F. and 5 parts ofpowoered silica aerogel was thoroughly mixed by repeated passage between the grinding rolls of a 3-roll mill. The mixture was then heated at a temperature of 250 1". for hours so as to impregnate the aerogel as completely as possible with the polymer, after which it was again passed between the rolls to assure thorough mixing. The mixture thus obtained is slightly grey, but nearly colorless, has a consistency corresponding to that of a soft grease, and is neutral to litmus. It remains of nearly the same body or consistency throughout a temperature range of from 40 F. to above 400 F. The mixture has a flash point of 620 F. and a fire point higher than 670 F. when tested by the method described in A. S. T. M. D92-33. It absorbs only 0.25 per cent of water on immersion over night at C. The arc resistance of the mixture is 130 seconds, according to tests carried out by the method of A. S. T. M. D495-41. and there is no carbon track remaining after passage of an electric are through the mixture. The mixture has a dielectric constant of 2.5 to 2.7 and a per cent power factor of 1.75 to 2.0 at 25 C. and 1000 cycles. The electric resistivity on avolume basis, in accordance with the method of A. S. T. M. D257-38, is 11x10" at 70 F.; 9.7 10 at 200 F.; and 8.5x 10" at 350 F. Samples of the mixture were contacted for 100 hours at a temperature of 300 F. with neoprene (a synthetic rubber), a film of ethyl cellulose (such as that used in lacquers), and rubber, respectively, without effect of the mixture on any of these substances. The mixture is compatible with benzene, gasoline. solvent naphtha, and carbon tetrachloride. It is insoluble in water. ethyl alcohol, and ethylene glycol. The mixture is, accordingly, satisfactory as an agent for sealing the shielded ignition cable terminal of a. spark plug. It may also be used wtih advantage to seal the terminal of an ignition cable where. it connects with a magneto, or to seal any portion 01 a high tension cable or other electric conductor that is positioned closely adjacent to a grounded metal.

Although the dielectric compositions provided by the invention consist essentially, and usually only, of a liquid polymer of an organo-silicone and a minor amount of an inorganic aerogel, it may be desirable in some instances to add other ingredients such as pigments or other coloring agents or a filler such as powdered mica. Any ingredients thus added should, of course, be ones which do not seriously impair the dielectric properties of the composition and should be present in minor proportion with respect to the liquid organo-silicone polymer.

Other modes of applying the principle of the invention may be employed instead of those explained, change being made as regards the method and products herein disclosed, provided that the steps or ingredients stated by any of the following claims or the equivalent of such stated steps or ingredients be employed.

I therefore particularly point out and distinctly claim as my invention:

1. A dielectric composition comprisin a major proportion of a liquid polymer oi an organo-sillcone wherein each organo group is a lower alkyl radical and intimately admixed} therewith an inorganic aerogel in a minor proportion corresponding to at least 0.1 per cent of the weight of the organo-silicone polymer and sumcient to render the composition appreciably more viscous than the organo-silicone polymer component thereof, which composition may be caused to flow at room temperature and conform to the shape of a cavity to be filled with the same.

2. A plastic dielectric composition comprising an intimate mixture of a major proportion of a liquid organo-silicone polymer wherein each organo-group is a lower alkyl radical, and a minor proportion of an inorganic aerogel in amount corresponding to at least 0.1 per cent of the weight of the organo-silicone polymer and suiiicient to render the composition appreciably more viscous than the liquid organo-silicone polymer component thereof, said liquid polymer being one wherein the ratio of carbon atoms to silicon atoms is between 1.75 and 4.

3. A plastic dielectric composition comprising as its major ingredient a liquid organo-silicone polymer wherein each organo-group is a lower alkyl radical, which polymer contains between 1.9 and 2.5 atoms of carbon per atom of silicon, and, intimately admixed and impregnated therewith, a minor proportion of silica aerogel in amount corresponding to at least 0.1 per cent of the weight of the organo-silicone polymer and sufllcient to render the composition appreciably more viscous than the liquid organo-silicone polymer component thereof.

4. A plastic dielectric composition comprising as its major ingredient a liquid organo-silicone polymer wherein each organo-group is a lower alkyl radical, which polymer contains approximately 2 atoms of carbon per atom of silicon, and silica aerogel intimately admixed and impregnated therewith in amount corresponding to between 3 and 15 per cent of the weight of said polymer.

5. A plastic liquid dielectric composition comprising as its major ingredient a liquid organosilicone polymer wherein each organo-group is a lower alkyl radical, which polymer contains approximately 2 carbon atoms per atom of silicon and is of a viscosity greater than 500 Saybolt seconds at F., and, as a minor ingredient, silica aerogel in amount corresponding to less than 10 per cent of the weight of said polymer.

6. A plastic dielectric composition comprising as its major ingredient a liquid organo-silicone polymer wherein each organo-group is a lower alkyl radical, which polymer contains approximately 2 carbon atoms per atom of silicon and is of a viscosity greater than 500 Saybolt seconds at 100 F., and, as a minor ingredient, silica aerogel in amount corresponding to between 5 and 7 per cent of the weight of said polymer.

7. A plastic dielectric composition comprising as its major ingredient a liquid dimethyl silicone polymer and, as a minor ingredient, silica aerogel in amount corresponding to between 3 and 10 per cent of the weight of said polymer.

8. A plastic dielectric composition consisting essentially of a liquid dimethyl silicone polymer of more than 500 Saybolt seconds viscosity at 100 F. and silica aerogel intimatel admixed therewith in amount corresponding to between 5 and 7 per cent of the weight of said polymer.

9. In an electrical device where a. high voltage electric conductor is closely positioned to a grounded metal, a seal between the conductor and the grounded metal adapted to exclude moisture and to prevent occurrence of a corona discharge, which seal is composed of a viscous dielectric composition consisting essentially of a liquid organo-silicone polymer wherein each organo-group is a lower alkyl radical and which organo-silicone polymer contains between 1.75 and 4 carbon atoms per atom of silicon, and a powdered inorganic aerogel intimately admixed therewith in amount corresponding to at least 0.1 per cent of the weight of the organo-silicone polymer and sufllcient to increase appreciably the viscosity oi the composition over that of the liquid organo-silicone polymer alone.

10. A terminal construction for a high tension cable where the latter connects with a shielded spark plug, comprising a grounded tubular metal shield extending from the spark plug and about said terminal, and, in the space between the terminal and the shield. a viscous dielectric composition consisting essentially of a liquid dimethyl silicone polymer of more than 500 Saybolt seconds viscosity at 100 F. and, intimately admixed therewith, powdered silica aerogel in amount corresponding to less than 10 per cent of the weight of said polymer.

11. A terminal construction for a high tension cable where the latter connects with a shielded spark plug, comprising a grounded tu bular metal shield extending from the spark plug and about said terminal and a viscous dielectric composition substantially filling the space between the terminal and the shield, which viscous dielectric composition consists essentially of a liquid dimethyl silicone polymer of more than 500 Saybolt seconds viscosity at 100 F. and, intimately admixed therewith, powdered silica aerogel in amount corresponding to between 4 and 7 per cent of the weight of said polymer.

SHAILER L. BASS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,188,007 Kistler Jan. 23, 1940 2,258,218 Rochow Oct. 7, 1941 2,258,220 Rochow Oct. 7, 1941 2,274,231 Behrman Feb, 24, 1942 2,173,539 Peters Sept. 19, 1939 1,760,583 Clark May 27, 1930 1,958,580 KasarJian May 15, 1934 2,252,496 Dutterer Aug, 12,1941 I 2,351,564 Wall June 13, 1944 

